The present invention is particularly applicable for heating steel as it progresses through a multiple zone heating furnace of the type having a preheat zone, a heating zone and a soaking zone with the products of combustion passing in a counterflow direction from the soaking zone to the entrant end of the furnace for discharge through a standard flue and it will be described in relationship to such use; however, the invention has broader applications and may be used with other materials in various throughput, fuel fired furnaces. A representative furnace arrangement is illustrated in prior U.S. Pat. No. 4,357,135. That patent relates to the concept of controlling the firing capacity in the preheat zone as a function of the firing capacity in the heat zone and the flue gas exit temperature. Such a control system has substantially increased the efficiency of reheating furnaces by decreasing the temperature of products of combustion flowing through the flue to the recuperator of the furnace. This efficiency is obtained by controlling the preheat firing in conformity with the actual production sensed by the firing rate of the burner or burners in the heating zone. Although such control is a substantial improvement in the operation of heating furnaces for steel and other metals, there is still substantial difficulty resulting from drastic changes in the production rate of the work flowing through the furnaces. Indeed, when the furnace experiences a delay, drastic changes in the heating operation of the furnace take place which require immediate, experienced intervention by furnace operators. This control by the operators is through adjusting the capacity of the various burners in each zone so that the workpieces will not overheat during a delay. Consequently, when the workpieces or work start progression through the furnace again, all the burners are out of equilibrium and require further attention by the operator. To alleviate this difficulty, experienced operators attempt to anticipate the movement of the workpieces through each zone of the furnace which result in a substantial increase in the heat energy carried by the products of combustion through the flue of the furnace even when controlled in accordance with prior U.S. Pat. No. 4,357,135.
When heating steel in a three zone furnace, there are manual control devices for each of the three zones. In practice, the heating zone is set to operate in a temperature range of approximately 2300.degree. F. to 2400.degree. F. This produces work to the soaking zone which is maintained generally in the range of 2350.degree. F. to 2400.degree. F. As long as the furnace is operated according to the expected production, controlling the individual zones in accordance with a preset temperature allows generally satisfactory results; however, the production rate through such furnaces can vary drastically. Indeed, a complete workpiece or work delay can occur. When the production rate is decreased, the temperature profile in each of the heating zones is changed. The temperature at the entrant end of the zone increases as a general function of the decrease in the production rate. If there is a delay, this entrant end temperature drastically increases. Thus, gas issuing from each zone has a higher temperature than would be experienced during normal operation of the furnace. When the production rate increases, the temperature profile longitudinally through each of the zones changes in the opposite direction. The temperature at the entrant end of the zone drops. This reduces the temperature of the products of combustion passing from the zone and thus results in an increased efficiency since the energy is being applied to the work and not passing directly through the zone. When the production rate is further increased, the zones may not be able to produce a desired temperature of the work for use of the work in the subsequent processing. These drastic changes in the profile of temperature from the exit end of the heating zones to the entrant end of the heating zones has required substantial and experienced operator intervention. Basically, the operators have attempted to determine an optimum production rate. Then the operator attempted to maintain this rate so that the burners in the individual zones did not have to be substantially adjusted. This manual arrangement, together with the control feature previously described in U.S. Pat. No. 4,357,135, has been successfully employed. However, fixing the rate of flow through the furnace is not a final solution to the efficiency problem. There is still a substantial need for operator intervention. The net result of this situation is decrease in efficiency because the operators maintain the zones hot enough to assure that the workpieces entering the soaking zone are at the desired temperature. Otherwise, the workpieces must be reprocessed, which drastically decreases the efficiency of the overall production facility. Another arrangement devised by operators for solving the control problem of a furnace designed to progressively heat steel has been to intermittently move the work through the various zones. In this manner, each zone is used as a batch heating zone; however, the loss of heat energy through the flue by this arrangement is often well over 50% of the energy. It has been known that the capacity of the burners in each zone could be manually operated to compensate for changes in production rate to increase the efficiency of the furnace as the workpiece is continuously moved through the furnace. However, there was no arrangement that was satisfactory to provide continuous and accurate control of each zone so that the overall heating efficiency of a multi-zone, fuel fired furnace could be increased for the purpose of saving fuel, which has drastically increased in market value.
Even when the individual zones were attentively adjusted by operators to compensate for supposed changes in production rate, two specific problems occurred. First, the operator could not envision actual changes in production rate within the zone itself. Control adjustments could be made only on actual stoppage of the work. When that happened, drastic changes in heating capacity of the burners were made. The burners were turned down. This did not generally control the heating profile from the entrant end of the zone to the exit end of the zone. The profile would generally equalize at some condition during stoppage. When the work continued on through the furnace, the operator would adjust the burners to a higher capacity, which may or may not be proper. As can be seen, such manual intervention in a fuel fired furnace could not be maintained over long periods and with unsupervised furnace operations. Consequently, even when training programs were established to change heating capacity of the furnace zones with drastic changes in production rate, such training did not provide a long term solution to optimizing the efficiency of the furnace.